This invention relates to ink jet printers, and more particularly to a mount for accurately positioning an ink jet printhead in a desired position for imprinting information or indicia on a surface of a series of articles (e.g. boxes or packages) conveyed past the printhead. The mount of this invention maintains a predetermined spaced relationship between a printing face of the printhead and the surface of the article or substrate to be imprinted.
In conventional ink jet print apparatus used, for example, for imprinting packages or boxes conveyed along a packaging line, each printhead typically has an array of orifice nozzles or ink outlet openings from which ink is expelled in the form of droplets. The droplets are expelled in a predetermined pattern toward a side of the carton or object (or other surface) to be imprinted such that upon the ink striking the carton surface, a predetermined indicia is imprinted on the surface. For example, such ink jet printing apparatus may be used to print a variety of information (indicia) on a carton or package, such as trademarks, lot numbers, serial numbers, production dates, shipping date information, bar codes, graphics, and other pertinent information. The ink nozzles or outlets of the printhead are in communication with a supply of ink. A programmable controller regulates the operation of the valves or other well known means for forming and expelling the ink droplets so as to cause the ink to be emitted from the nozzles according to a preselected pattern so as to imprint the desired information or indicia on the surface to be imprinted.
Generally, the ink nozzles or outlets are located on a print face of the printhead with this print face being spaced a short distance (referred to as the orifice-to-substrate distance or the standoff distance) from the surface to be imprinted. Thus, ink emitted from the ink jet nozzles must travel from the printface to the surface to be imprinted across the orifice-to-substrate distance (which is typically a small fraction of an inch) to form the ink droplets of the desired shape and size and to form an ink droplet pattern of the desired resolution so as to form the indicia to be imprinted. Typically, an ink droplet grows wider as it travels from the outlet opening or nozzle to the surface to be imprinted. As the width (diameter) of the droplet increases, a larger or irregular dot will be printed upon the surface impacted by the droplet. As the droplet gets wider, the outer edges of the printed dots loose precision or resolution, and the quality of the printing deteriorates. Likewise, if the spacing of the printhead is too close to the surface to be imprinted, the droplets may not sufficiently overlap and thus the quality of the indicia to be imprinted will be adversely affected. Accordingly, the orifice-to-substrate distance of the printhead is a critical parameter in maintaining the quality of the indicia to be imprinted.
Reference may be made to U.S. Pat. No. 4,378,564 for a disclosure of a typical ink jet printing system for imprinting packages in a packaging line.
In a typical packaging line, the surfaces of cartons or other articles to be imprinted move along a conveyor path past one or more ink jet printheads. For example, the surfaces to be printed are typically the side of a carton or package being conveyed along a conveyor belt past an ink jet printing station. Of course, there are typically a series of packages one after the other that are conveyed past the print station. It is common to have one or more printheads on opposite sides of the conveyor path so that the printing can be done on opposite faces of the packages.
To guide the packages into position relative to the printhead or printheads, guide rails are commonly provided along the conveyor so as to orient and position the package on the conveyor line so that the surfaces of the packages are positioned in predetermined planes relative to the line of movement of the conveyor and relative to the print station as they move therepast. The guide rails confine the packages between them as they are transported by the conveyor.
Packages in such a packaging line are typically of the same nominal size, but they may vary in width, height, and length due to manufacturing tolerances. In addition, there may be irregularities or undulations (bulges) in the faces of the packages to be printed. Therefore, the span between the guide rails must be great enough to accommodate the widest of the packages as permitted by a range of package tolerances. Smaller packages within this permitted range of tolerances may not contact the rails. Accordingly, although the guide rails position the packages at nominal distances from the printhead, those distances vary as the packages to be imprinted are conveyed past the printheads. Further, it is generally a desire of such conveyor lines that the widest range of package tolerance possible be accommodated by the printheads. In order to achieve optimal printing quality and resolution, it is necessary that the ink jet printhead be mounted to move toward or away from the conveyor line such that the printing head may be positioned to have the optimal orifice-to-substrate distance from the surface to be imprinted, regardless of size variations of the packages to be printed and regardless of surface undulations or irregularities of the package.
Means have been developed for reducing the orifice-to-substrate distances of printheads and for maintaining a tighter tolerance of the orifice-to-substrate distances so as to enhance print quality and resolution. One such means is shown in co-assigned U.S. Pat. No. 4,814,795, which is herein incorporated by reference. In this last-mentioned patent, printheads of such ink jet printing apparatus are spring biased toward the surface to be printed so as to accommodate a range of package sizes conveyed past the printhead and so as to maintain a substantially constant orifice-to-substrate distance between the ink jet nozzles and the surface of the package to be printed. While this prior art printhead mount worked well for its intended purpose, this printhead mount required an additional separate slide mount to support the printhead. The slide mount included horizontal rods on which slide bearings were mounted which in turn carried the printhead for transverse movement of the printhead toward and away from the surface of the package to be imprinted as the package was conveyed past the printhead. The perpendicular travel of the mount relative to the conveyor caused the slide rods to be subject to being bent or peened in the operative life of the slide mount. Of course, an imperfection in one of the slide rods would impede the proper operation of the slide and would have a negative effect on print quality. Further, as the speed of the packages conveyed past the printhead was increased, the engagement of the printhead by packages at such higher speeds resulted in sudden impact loads being applied to the printhead and/or the mount which resulted in bouncing of the printhead and the mount. In turn, these impact forces and the consequent bouncing of the printhead resulted in poorer quality printing and resolution, as compared to printing at slower speeds. In addition, the impact loads, under certain conditions, could cause the printhead to de-prime (i.e.., lose its ink supply) such that is would not properly print.
U.S. Pat. No. 5,101,224, which is also assigned to the same assignee as the present invention and which is also incorporated herein by reference, solved some of the aforementioned problems. Specifically, this last noted prior art patent provided an ink jet printhead support that carried the printhead on a pivotally mounted support that was biased inwardly toward the surface of a package or article by means of a torsion spring. The support permitted vertical and horizontal positioning of the printhead, and eliminated the above noted slide rods, however, this printhead support was complex in construction and the action tended to skew the print, depending on the placement of the package relative to the pivot point. In addition, this prior art printhead support was not sufficient for higher resolution printheads.